Ortho Elimination from Even-Electron Ions

2 Ortho Elimination from Even-Electron Ions 

As with the McLafferty rearrangement and the retro-Diels-Alder reaction before, the occurrence of the ortho elimination is not restricted to molecular ions. It may equally well proceed in even-electron species. 

Example Examination of the mass spectrum of isopropylbenzoic acid reveals that the molecular ion, m/z 164, as well as the [M-CHs] ion, m/z 149, eliminate H2O via ortho elimination yielding fragment ions at m/z 146 and 131, respectively (Fig. 6.45) [178]. The [M-CH3-H20] ion, a homolog of the benzoyl ion, decomposes further by loss of CO, thereby creating a fragment that overall corresponds to [M-CHb-HCOOH]- , m/z 103. 

Example The molecular ion of l,2-bis(trimethylsiloxy)benzene, m/z 254, undergoes methyl loss by Si-C bond cleavage as typically observed for silanes (Fig. 6.46). Rearrangement of the [M-CHb] ion then yields [Si(Me)3], m/z 73 (base peak). This is not an ortho elimination with concomitant H transfer as defined in the strict sense, but the observed reaction is still specific for the orthoisomer [169,179]. hi the spectra of the meta- and para-isomers the [Si(Me)3] ion is of lower abundance, the [M-CHb] ion representing the base peak in their spectra. Moreover, the m/z 73 ion is then generated directly from the molecular ion which is clearly differmt from the two-step pathway of the ortho-isomer. 

Note Trimethylsilyl (TMS) derivatives are frequently employed to volatize alcohols [179,180], carboxylic acids [181,182], and other compounds [183] for mass spectrometry, and for GC-MS applications in particular. The El mass spectra of TMS derivatives exhibit weak molecular ion peaks, clearly visible [M-CHs] signals and often [SifMels] , m/z 73, as the base peak. 

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